Yes, rivalries are created by geography and proximity - teams that play each other a lot are naturally better rivals. But that doesn't always create rivalries. The Twins and Tigers never had the rivalry with each other the Twins and White Sox had, despite competing for several division titles. They never disliked each other enough. Other factors - which can include blown calls by umps/refs, physical play/fights, free agent signings, draft picks, etc - can all add to a rivalry. If Brady Aiken ever makes it to the majors, his games against the Astros are going to mean something more to his fan base simply b/c of how Houston treated him last year. It adds something fun for the fan bases.